1. Field of the Invention
The present invention relates to the non-destructive thickness measurements of the paint and coating thickness for graphite/epoxy composites, and other poorly conductive substrates. In particular, the invention may be used to measure the dry paint film thickness applied by conventional or electrostatic spraying of primer, paint, or other coatings on a poorly conductive surface.
2. Statement of the Problem
In the spray application of many paints, it is difficult to maintain a uniform thickness application of the primer or paint. The difference in paint film thickness results in appearance changes in which the substrate shows. As a result of the substrate showing through, users tend to apply primer and paint thicker than specified on graphite/epoxy composites. Present methods for measuring dry film thickness on metal substrates do not work on graphite/epoxy composites. There is no good and accurate non-destructive method or device for quickly and accurately measuring the primer, paint and coating thickness on graphite/epoxy composites. This is because the eddy-current thickness (skin depth) in graphite is greater than most paint thickness (3-6 mils). Over metal substrates the eddy-current paint meters (f.about.5 MH.sub.z) have skin depths much less than this paint thickness. By raising operating frequency to .about.10.sup.9 Hz (X-band) we solve the skin depth problem, but encounter another. It is difficult to create the neat circular eddies--with high Q and insensitive to edge effects--that one readily obtains at low frequencies with a simple eddy "pancake" coil. To solve this second problem, we employ a choke flange (normally used to couple waveguides end to end) to create a standard (well-defined) current distribution on the Surface Under Test (SUT) i.e., the painted graphite. Incidentally, this non-circulating (non-eddy) current distribution responds mainly to the TOP layer of unidirectional graphite usually used in aerospace structures.
There is a need for a dry film thickness measurement instrument and method for graphite/epoxy composites which does not destroy the paint and coating and accurately measures the dry film thickness on graphite/epoxy composites surfaces.
3. Object of the Invention
One of the objects of the present invention is to provide a measuring instrument which is able to measure thickness of the primer, paint, and coating on metal and graphite/epoxy composites without destroying the dry film layer. To this end, the present invention uses a measurement cavity that oscillates at a fixed microwave frequency dependent on the apparent length of the cavity.
However, it is difficult for users to correlate a given frequency change with a thickness measurement. Therefore, another object of the invention is to provide a measuring instrument which converts the frequency changes due to the apparent length change of the cavity into a thickness measurement.